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Swiss cheese model — a combination of ‘interventions’ experts are suggesting to combat Covid

According to the Swiss cheese model, no intervention is 100% effective but multiple layers of interventions can make up for individual deficiencies.

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New Delhi: Several public health experts are hailing the ’Swiss Cheese model’ as an effective way of combatting the spread of Covid-19 on social media.

In a tweet, Nicholas A. Christakis, Sterling Professor of Social and Natural Science at Yale University in the US, explained that the model suggests combining ‘contact reduction interventions’ with ‘transmission reduction interventions’ to stop the spread of the viral disease.

Contact reduction interventions involve shutting down schools, lockdowns and night curfews while transmission reduction interventions include washing hands, wearing masks, faster and sensitive testing. These are referred to as non-pharmaceutical interventions since they don’t involve vaccines or medicines.

According to the model, each intervention represents a slice of Swiss cheese. Like slices of Swiss cheese have holes in them, no intervention is foolproof alone. However, when a set of them like physical distancing, ventilation, masks, hygiene, fast sensitive testing, contact tracing and surface cleaning are implemented together, they are effective in bringing the outbreak under control.

Thus, multiple layers of intervention can be successful in making up for each intervention’s individual deficiencies.

Christakis further wrote, “In order to stop the spread of the SARS-Cov-2 and substantially reduce the risk of the Covid-19 epidemic in a family, firm or nation, one just needs enough layers of Swiss cheese, but not necessarily all of them.”

Also read: US reports first case of Covid reinfection with more severe symptoms second time

‘No measure 100% effective’

Several other virologists and epidemiologists agreed with Christakis’s assessment.

Virologist Ian M. Mackay even created a visualisation of the model, as mentioned by Christakis in his tweet mentioned above.

Dr Siouxsie Wiles, a microbiologist and associate professor at the University of Auckland in New Zealand, said, “No measure is 100% protective, hence we add layers like masks & hand washing & contact tracing apps = the ’Swiss cheese model’ to protect us from Covid-19.”

A physician from Canada, Dr Jennifer Kwan, also endorsed this model of Covid-19 ‘prevention’.

Giridhar R. Babu, professor and head of department, lifecourse epidemiology, Public Health Foundation of India, told ThePrint, “The Swiss cheese model is nothing but a proposition that no single intervention is completely successful. I do agree that countries need a combination of several factors for successful Covid-19 response.”

He added, “I have maintained that one must follow 3Ws, which are watch your distance, wear a mask, wash your hands and avoid 3Cs which are closed spaces with poor ventilation, crowds and close contact settings. If you must, then one may call it as six layers of cheese. It is all upto how one wants to simplify the message.”

Also read: Laxity, low seroprevalence & reliance on antigen tests — how Kerala’s Covid battle came undone

Studies have recorded success of NPIs

The ‘Swiss Cheese model’ was first conceptualised by James Reason, a professor at the UK’s Manchester University in 2000.

Reason introduced this concept to shed light on systemic failures and suggested that mishaps could be prevented “by a series of barriers”. A barrier by itself could have holes, but when barriers are combined these holes open and close at random. This significantly decreases the chances of human losses.

Several studies have attempted to estimate the effects of such non-pharmaceutical interventions (NPI) in combatting Covid-19.

A study assessing NPI and its effectiveness in Europe found that “major non-pharmaceutical interventions — and lockdowns in particular — have had a large effect on reducing transmission”.

Another study, conducted in China, revealed that the number of cases would have increased 67 times without NPIs. The study said, “We estimate that early detection and isolation of cases prevented more infections than did travel restrictions and contact reductions, but that a combination of non-pharmaceutical interventions achieved the strongest and most rapid effect”.

Also read: 2 parameters will decide India’s Covid vaccine priority — risk of exposure & of severe disease


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